Image capturing by means of technical means is increasing as more and more people are walking around with devices capable of capturing still images and/or motion video. Moreover, captured images, still images and/or motion video, are increasingly used in monitoring and surveillance systems. In order to capture image information for a particular use, the exposure of the image sensor to the light it is to capture has to be precisely controlled. In order to make it easier for an end user to capture usable images, automatic schemes for setting the proper exposure has been developed. Such automatic schemes for setting the exposure are of particular importance in systems continuously capturing still images and/or motion video during an extended period of time. For example, the lighting conditions may vary substantially during any extended time period, in particular during evenings, mornings, and time periods extending over an entire day. The lighting conditions may also vary substantially during shorter time periods due to changes in weather, changes in cloud cover, and changes to the illumination of the scene, e.g. electrical lights are turned on or off.
In some cameras the exposure settings for the scene in front of the camera are automatically calculated based on inputs from separate light meters mounted in the cameras. In other cameras the exposure settings for the scene are automatically calculated based on image data captured by the image sensor of the camera. One example of the use of the image data from the image sensor for calculating exposure settings is presented in U.S. Pat. No. 8,446,480 B2 (KALEVO). This patent describes a method and an apparatus for exposure control. A histogram of the number of sensor elements in an image sensor that receive light at a specific intensity is determined. Moreover cost functions expressing the performance of the image sensor as a function of received light intensity is available for a plurality of exposure times. Then an exposure time is determined based on this histogram and the cost function. The teachings of the documents relates to finding the exposure for the capture of a still image. One problem with the above method is that the method is designed for use in capturing single still images and that the teaching of it therefore does not address problems related to capturing motion video or still images captured as part of a series representing a camera view over an extended time period.